Classically, the heart is viewed as consisting of two pumps in series, with the output of one ventricle becoming the input to the other ventricle. Thus, one ventricle could only influence the other ventricle by changing it's output, thereby, eventually changing the input to the other ventricle. However, the anatomical association between the ventricles suggest that a direct myocardial relationship exists. Changes in the volume of one ventricle have been shown to effect the diastolic compliance and shape of the contralateral ventricle. In isolated heart preparations, changes in left ventricular volume and free wall integrity have been shown to alter right ventricular systolic pressure. Experimental and clinical data suggest that ventricular interactions are also important in the pathophysiology of pressure overload and volume overload. This study will examine biventricular mechanics in the intact dog by simultaneous measurements of left and right ventricular pressures and volumes. A computer algorithm will use the positional information obtained from radiopaque markers implanted on the ventricular endocardial surface to calculate the ventricular volumes. Using this preparation the effects of respiration, posture, pericardial pressure, pressure overload, and volume overload on biventricular function will be examined. The information obtained in this study will provide new insight into ventricular mechanics in normal and pathological conditions, and will improve the understanding of the mechanisms which produce right ventricular dysfunction.